Developments and Effects in South Tyrol and the Alps
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Snow DOSSIER Developments and effects in South Tyrol and the Alps Michael Matiu Snow Snow defines the winter landscape, provides vital water resources for ecosystems and agriculture and creates jobs. But climate change is threatening snow. How has snow changed in South Tyrol and the rest of the Alps? What is expected in the future and what will the consequences be? Michael Matiu, Institute for Earth Observation, with the help of Giacomo Bertoldi, Claudia Notarnicola, and Marc Zebisch The intricate beauty of snow crystals is created as they travel through the atmosphere - environmental conditions determine their final shape. (Foto: Bresson Thomas) What is snow? Snow is the most common form of solid precipitation. It consists of ice crystals that are first formed in the atmosphere then grow further while falling. Their final size and shape depend on atmospheric conditions, especially temperature, humidity, and wind. On the ground, snow accumulates to form the snow cover. The texture, size and shape of the snow grains change over time depending on surrounding conditions, such as temperature which causes melting and refreezing, wind transport or subsequent snowfall causing compression. Over the winter season, the snow cover accumulates a complex multi-layer structure, reflecting the weather conditions during and after each snowfall. The snowpack is a complex structure, its layers tell the story of the winter from the first to the most recent snowfall. (Foto: David Newman) When does it snow? Snow feeds on moisture and cold temperatures in the atmosphere. Snow reaches the ground if surface temperatures are below 0°C. In a few specific cases snow can also reach the ground in temperatures of up to +5°C. Complex terrain, such as in South Tyrol, modulates the weather. Temperature generally decreases with altitude. The mountains influence the flow of air masses. The windward side receives more precipitation at higher altitudes. Leeward, air masses are drier resulting in those regions receiving less precipitation and consequently, less snow. This effect can lead to some regions, such as the Venosta valley, receiving very little moisture from the air masses. Altogether, this results in a complex spatiotemporal distribution of snow cover, where the main feature is more snow with increasing altitude. How is snow cover measured? Recording scientific measurements of snow cover is rather straightforward and only requires a graduated stick or rod, which allows the depth of snowfall (fresh snow; height relative to empty surface) and snow depth (height relative to ground) to be measured. Such measurements date as far back as to 1787 in Torino and 1882 in Rovereto. In South Tyrol, extensive manual observations started in 1980. Recently, automatic sensors were also deployed in higher elevations; however, their time series are not yet sufficient for longer climatological assessments. In recent decades, remote sensing has offered an alternative to accu- rately measure snow cover area, snow cover duration and snow type (wet/dry). The main benefit of remote sensing is that it covers the whole altitudinal gradient, although data relating to the past is available only for a short period. Hydrologically, the amount of water stored in the snow cover, the so-called snow water equivalent, is a key variable. However, its manual measurement is very labour intensive and the automatic measurement very costly, which is why it is not as widely observed as depth of snowfall or snow depth. 80 80 80 80 80 80 80 80 80 Trend analysis of snow 40 40 40 40 40 40 40 40 40 depth for all sites in South 0 0 0 0 0 0 0 0 0 N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M Racines di Dentro / Terme di Brennero / Brennerbad, Diga di Vizze / Wehr in Pfitsch, Diga di Neves / Stausee Neves, Tyrol that have more or Melago / Melag, 1915m Plata / Platt, 1130m Innerratschings, 1260m Ridanna / Ridnaun, 1350m Pennes / Pens, 1487m 1330m 1365m 1860m Fundres / Pfunders, 1150m 80 80 less complete records 40 40 for the last four decades 0 0 N D J F M A M N D J F M A M (1981-2020). The change in S.Valentino alla Muta / S.Maddalena in Casies / St.Valentin auf der Haide, 1525m St.Magdalena in Gsies, 1403m snow depth is determined 80 80 40 40 via a linear regression 0 0 analysis of monthly mean N D J F M A M N D J F M A M Roia di Fuori / Ausserrojen, Diga di Valdaora / Stausee 1833m Olang, 1057m snow depth by year. 80 80 Negative trends in red, 40 40 positive trends in blue. 0 0 N D J F M A M N D J F M A M Slingia / Schlinig, 1690m Sesto / Sexten, 1310m 80 80 40 40 0 0 N D J F M A M N D J F M A M Monte Maria / Marienberg, 1335m Luson / Lüsen, 981m 80 40 Legend 0 Example: December for N D J F M A M S.Cassiano / St.Kassian / S.Ciascian Mazia / Matsch, 1570m 80 80 40 40 Monthly mean snow depth [cm] snow 0 0 N D J F M A M 1980 1990 2000 2010 2020 D Diga di Gioveretto / Stausee Zufritt, 1851m 80 80 80 80 80 80 80 80 80 40 40 40 40 40 40 40 40 40 0 0 0 0 0 0 0 0 0 N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M N D J F M A M Fontana Bianca / Weissbrunn, Diga di Zoccolo / Zoggler S.Pancrazio / Stausee Nova Ponente−Maso d.s. / Funes / Villnöß / Funès (Flitz/ Ortisei / St.Ulrich / Urtijëi, La Villa / Stern / La Ila S.Cassiano / St.Kassian / 1900m Stausee, 1144m St.Pankraz, 810m Anterivo / Altrei, 1209m Deutschnofen−Oberhof, 1415m Valluzza), 1350m 1180m (Badia), 1390m S.Ciascian, 1545m Have snow depths changed in South Tyrol? Over the last 40 years snow depth has decreased at most sites in South Tyrol, but differences depending on month, elevation, and location have been recorded. For the 28 locations with more or less complete records from 1981 to 2020, decreases were observed in winter (December to March) at an altitude mainly below 1500m, while above that mark, increases and decreases were equally distributed. Moreover, there were spatial gradients, negative trends were observed mostly in the north and east, while positive trends were observed in the south and west. For example, in the western part of the province, Slingia/Schlinig, situated at an altitude of 1690m, had a mean snow depth for February which increased from 48cm to 63cm, while at 1860m, Diga di Neves/ Neves Stausee, in the north-east, the snow depth decreased from 86cm to 66cm. At the end of the season, that is April to May depending on elevation, no positive trends were observed. In fact, in most locations snow has been disappearing on average in the last month of the snow season. Today, April is on average snow-free below 1500m, while 40 years ago, 7 to 24cm of snow was not unusual, such as in Sesto/Sexten or Pennes/Pens. The main limitation of these results is that they are restricted to locations below 2000m, because above that altitude, no long-term observation records exist in South Tyrol. What’s the situation in the rest of the Alps? The changes in South Tyrol are similar to what has been observed in the southern part of the Alps. The Alps are located in a transitional area influenced by three main climate forcing zones: the Atlantic Ocean, Mediterranean Sea, and European continent. The main climatic boundary is found along the central ridge which separates the north from the south, followed by an east-west-gradi- ent. This affects temperature and precipitation especially in winter when the south receives on av- erage 1/3 less precipitation than the north, which translates to approximately 20-30% less snow. An analysis of more than 800 stations throughout the Alps identified reductions in winter snow depth for most of the stations below 2000m in the period 1971-2019. Moreover, the decreases in the south were more prominent than in the north. Above 2000m, a high variability in trends was observed, from strong decreases in snow depth to strong increases. However, observations above 2000m over such long periods are sparse, so no alpine-wide statements are possible. In fall and spring, all stations in all regions showed the most pronounced decreases in snow depth. Average area that is snow covered Observation Climate model projected change Climate model projected change (remote sensing) (without mitigation) (global warming below 2°C) (snow cover fraction) in winter 2000−2020 2071−2100 2071−2100 (December to February) and spring (March to May) for the present (2000-2020) and projected changes for the future (2071-2100). Winter Present observations are based on 20 years of satellite observations, and future conditions based on an ensemble of regional climate models, which have been bias corrected using observations. The maps have a horizontal resolution Spring of 12km, which corresponds to the resolution of the current generation of regional climate models.